An electric double-layer capacitor has a structure in which a pair of polarizable electrodes made of activated carbon are disposed as a positive electrode and a negative electrode to face each other via a separator. Each polarizable electrode is impregnated with a solution of water-soluble electrolyte or a solution of electrolyte in non-aqueous solvent, and the each polarizable electrode comes in contact with collecting electrodes, respectively.
Improvements of performances such as resistance, low temperature characteristics and life among characteristics of the electric double-layer capacitor are noticeably influenced by pore diameters of the activated carbon for use as each polarizable electrode. It is considered that, as pores of the activated carbon, the mesoporous of larger than 2 nm but 20 nm or less play a more important role than the microporous of 2 nm or less (e.g., Non-Patent Documents 1 and 2). Concerning pore diameters or pore volumes of the activated carbon, a large number of patent applications have been filed (e.g., Patent Documents 1 to 7). In addition, it is considered that a capacitance of the electric double-layer capacitor is substantially proportional to surface area of each polarizable electrode, and the activated carbon having a large specific surface area is usually used as a carbon material for the electric double-layer capacitor (Patent Document 1).
The activated carbon is usually produced by carbonizing a carbonaceous material at a temperature of 800° C. or less and then subjecting the carbonized material to an activation treatment. Here, the activation treatment includes, for example, a method of heating the carbonized material at 600° C. to 1000° C. in an atmosphere of water vapor, carbon dioxide or the like; and a method of mixing the carbonized material with zinc chloride, potassium hydroxide and the like and heating the mixture in an inactive atmosphere. In this activation process, a large number of pores suitable for adsorption are generated in the surface of the carbon material produced in the carbonization step. Further, the polarizable electrodes are prepared by a method comprising the steps of adding a conductive agent and a bonding agent to the activated carbon and subjecting the mixture to kneading and rolling; a method comprising the steps of: mixing the activated carbon with non-activated resins and sintering the mixture; or the like.